(Adapted from the Applicant's Abstract) The purpose of the core is to provide the resources and expertise for the efficient generation and delivery into embryos of recombinant expression viruses (adenovirus and retroviruses) containing genes of interest to the project investigators. (End of Abstract) This core provides the critical adeno-viral and retro-viral support for the projects in this proposal. Most of the projects propose to either use lineage tracing or expression of certain genes utilizing either adeno-viral or retro-viral transfection into mouse or chick embryo cultures. This core is central to the experiments outlined in the various projects. The principal investigator and co-investigators have experience in the adeno-viral and retro-viral techniques proposed. Their approach is straight-forward and appropriate. They will maintain cell lines necessary for these experiments and generate the high titer viral stocks necessary for the experiments. They propose to make the core laboratory available to the investigators for them to learn as well as provide this service. The plans to provide direct infection of whole embryos in culture (mouse and chicken) are reasonable, and the investigators have experience with such techniques. There are overall no glaring difficulties with the proposed approach. The types of experiments proposed are now rather routine and will be useful for the various experiments outlined in the individual projects. The ability to infect specific tissues in whole embryo cultures is rather innovative, although it has been used successfully by other investigators. Nevertheless, it is not a common skill and it is certainly a strength that this core will have that ability. The prinicipal investigator and co-investigators have good experience, as they have demonstrated, utilizing retro-viral and adno-viral techniques. They have done these sorts of experiments for other purposes successfully, and should have no difficulty in performing the tasks outlined. Dr. Kubalak has had experience with mouse embryo culture and this will certainly be an asset to the program project grant. The environment for the core is strong and they appear to already have all the necessary equipment and technology to provide the core services outlined. There are no problems with the environment. This is a very important core for this program project and appears to be designed appropriately without any major flaws. The core has many strengths and will be a critical one for most of the projects. The availability of Dr. Mikawa as a consultant adds furthur strength to the ability of this core to troubleshoot and be effective.